1. Field of the Invention
This invention relates to the drilling of wells into high temperature subterranean formations containing geothermal fluid. More particularly, this invention relates to an improved drilling fluid composition and method of use thereof in the drilling of wells into high temperature geothermal formations.
2. Description of the Prior Art
Drillling wells into subterranean formations containing a geothermal fluid presents many problems concerning the choice of a drilling fluid. Conventional aqueous base drilling fluid containing water, clay and various additives is generally used in drilling the first two to four thousand feet of hole through the usually somewhat incompetent strata above the geothermal formation. Casing is cemented in place over this portion of the hole and drilling is continued using any one of a number of drilling fluids. Problems arise at lower depths when the drill bit penetrates those formations containing a geothermal fluid, which formations often lie several thousand feet below the surface. Such formations are generally highly porous and permeable. If conventional drilling fluid is used, the weight of the high gravity column of drilling fluid in the well is so great that a considerable amount of the fluid invades the formation. This necessitates use of a large additional volume of fluid and results in partial plugging of the formation by the clay and other solids in the fluid. When the well is subsequently completed, this plugging decreases the amount of geothermal fluid which can be produced via the well. If a relatively light-weight drilling fluid, such as air or another gas, is used, a high rate of circulation of the drilling fluid through the system is required to circulate cuttings out of the hole. This high rate of circulation of the drilling fluid together with the fluids produced from the formation, result in corrosion and erosion problems. The high velocity gaseous fluids erode the unconsolidated portions of the formation to such an extent that hole stability cannot be achieved. In many cases, even the drill pipe is eroded.
One solution to these problems has been to drill formations containing a geothermal fluid with aqueous gas-containing drilling fluid. Use of this light-weight, gas-containing drilling fluid comprising a mixture of water, a gas such as air, and optionally a foaming agent, results in increased penetration rates, reduced bit wear and reduced formation damage due to erosion by the drilling fluid as compared with air. However, the water in the drilling fluid easily invades the formation necessitating large quantities of fluid for make up and also can cause plugging. It would be desirable to utilize a light-weight, gas-containing drilling fluid requiring less make up and causing less damage to the formation due to invasion thereof as well as having improved ability to remove cuttings from the hole.
U.S. Pat. No. 2,650,197 issued Aug. 25, 1953 to Rahn and U.S. Pat. No. 3,385,789 issued May 28, 1968 to King describe clay-containing aqueous drilling fluids containing lignite. U.S. Pat. No. 2,552,775 issued May 15, 1951 to Fischer and Cook and U.S. Pat. No. 2,718,497 issued Sept. 20, 1955, to Oldham and Kropa disclose the use of alkali metal and ammonium water-soluble salts of a polymer of acrylic acid in clay-containing aqueous drilling fluids. While the techniques disclosed in these patents have certain advantages, need remains for a drilling fluid composition and method for drilling wells into subterranean geothermal formations that provide reduced penetration of the drilling fluid into the formation and improved wall-building and cuttings-carrying capacity.
Accordingly, a principal object of this invention is to provide a light-weight drilling fluid and method of use thereof suitable for drilling a subterranean formation containing a geothermal fluid.
A further object of the invention is to provide such a composition and method for drilling formations containing high temperature wet stream.
Another object of the invention is to provide such a composition and method which reduces the damage to the formation due to the invasion of drilling fluid into the formation surrounding the well.
Still another object of the invention is to provide such a composition and method having improved wall-building and cuttings-removing properties.
An additional object of the invention is to provide such a composition which does not contain clay.
Other objects, advantages and features of the invention will become apparent from the following description and appended claims.